A balanced microphone (as almost all decent quality ones are) uses three connections - this reduces noise and interference drastically. If you connect an un-balanced microphone to it, you simply use one of the inputs and ground - connecting the other input to ground. If it connects via a stereo jack socket, plugging a mono jack in will do this automatically. If it's an XLR connector, an unbalanced microphone will be wired to it as well.